A black hole is a volume from which photons, or any matter, can not escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

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Why don't the black holes appear black in color in images of galaxies taken from HST?

According to NASA a black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area According to the documentary Space Unraveling The Cosmos about ...
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Can the mass within the event horizon of a black hole interact gravitationally with the mass outside the event horizon?

If so, gravitons and their fields, unlike photons, must be able to cross the event horizon freely in both directions. If not, the observed mass of a black hole must depend only on the particles ...
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How can the energy of a spinning black hole be converted into magnetic field?

I've read about the Blandford-Znajek process, and what I understood is that the black hole is treated as if it were a conducting sphere spinning rapidly with an accretion disk around it, and the ...
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Using a black hole as a mirror?

http://www.youtube.com/watch?v=ENd8Sz0AFOk The YouTube video is a good example how the gravity of this merging binary black holes bend light around themselves.
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Coordinate Singularity in Metric

Suppose I have some metric $$ds^2=g(t)dt^2+\frac{1}{r}dr^2$$ which has a singularity at $r=0$. However, if I make the coordinate transformation $u=\frac{1}{r}$, then I get: $$ds^2=g(t)dt^2+r^3 ...
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Do black holes really exist, theoretically?

Do black holes really exist? Casting aside all the experimental proof, how do we arrive at the idea of a black hole theoretically using any metric (say Robertson-Walker) other than the Schwarzchild ...
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Why can't our expanding universe be an imploding universe that appears expanding relative to us? [closed]

Galaxies moving away from us is our proof of an expanding universe. However, if the universe was contracting towards a black hole we would see a galaxy ahead of us accelerating faster towards that ...
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Why does an evaporating black hole always stay a black hole?

Stars can only collaps and form black holes if their masses are above the Chandrasekhar limit, $M>M_{\rm Pl}^3/M_{\rm hydrogen}^2$. When the universe eventually cools down enough, the black holes ...
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Closed timelike curves in the Kerr metric

I just read in Landau-Lifshitz that the Kerr metric admits closed timelike curves in the region $r \in (0, r_{hor})$ where $r_{hor}$ is the event-horizon ( I am talking about the case $|M|>|a|$ ...
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Kerr-Newman Black Holes' Phase Transitions in De Sitter Space

At $J=km^2$ Kerr black holes' specific heat shows infinite discontinuity. Therefore as Kerr black holes' angular momentum gets close to $km^2$ where $k$ is determined by Kerr-Newman metric and $m$ is ...
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Conformal infinity in the Hawking-Hunter-Taylor-Robinson metric

I have been trying to follow some of the computations of this paper: http://arxiv.org/abs/hep-th/0408217 and particularly I couldn't derive the asymptotic form of the Kerr-AdS background (3.27) using ...
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Minimum gravity required to suck light? [closed]

I was studying about gravitational forces of black holes when I came to a thought of what is the minimum gravity required to capture light in vacuum?BY capturing I mean the inability of light to ...
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If we increase the mass of a black hole by a factor of $k$, by what factor will the surface area of the event horizon change?

If we increase the mass of a black hole by a factor of $k$, by what factor will the surface area of the event horizon change? Given any number of identical black holes each with mass = $\mathrm{m_0}$ ...
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Is white hole has minimum entropy in a region?

From here I know black hole has maximum in the region, how about white holes, can we deduce white hole has the minimum entropy in the region?
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49 views

Spaghettification on an atomic scale?

Spaghettification occurs when an object approaches a singularity. As one comes close enough to the singularity, the gravity at the feet (if this is a human) is greater than that at the head, ...
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Surface gravity of Kerr black hole

I'm going through Kerr metric, and following the 'Relativist's toolkit' derivation of the surface gravity, I've come to a part that I don't understand. Firstly, the metric is given by ...
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With gravitational lensing could we see our selves from a right angle?

When looking at a black hole the ability to see ourselves is compromised by the small window for the light trajectory to come back to us. The light from the Earth that is bent by (almost) 180° and ...
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Are Black Holes really spinning? [duplicate]

I've had this question: are black holes really spinning? Well if they are not spinning then why would the galaxy around them spin at all? They must spin! Or? Do they spin like a ball? What if they ...
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A star starts at rest at infinity, how do you calculate the kinetic energy of the star when it crosses the event horizon of a black hole? [duplicate]

A star that begins very, very far away from a black hole (infinity) is brought in by the black hole's gravitational pull. How do you calculate the kinetic energy of the star as it passes the point ...
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The time it takes for a black hole to devour a star

It has been suggested here: How long does it take a black hole to eat a star? that it can take (at best) a rather short amount of time for a supermassive black hole to eat a star as viewed by a ...
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In what other fields of physics does “math break down”? [closed]

I've heard numerous articles and videos state that when it comes to black holes and singularities our "math breaks down". Is there any other areas in physics where math similarly "breaks down", where ...
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How do we measure Schwarzschild coordinates?

In special relativity, we make a big fuss about setting up inertial frames of reference, and then constructing coordinate systems using networks of clocks and rulers. This gives an unambiguous ...
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How much energy is released by an evaporating black hole in the last second of its life?

From what I understand, due to Hawking radiation, black holes lose mass in the form of energy (electromagnetic radiation), with these characteristics: The larger the black hole, the less energy it ...
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How long does it take a black hole to eat a star?

I presume the answer is that it depends on the mass and size of the star and black hole and how they approach either other, but I was wondering if somebody could provide some rough bounds (e.g. hours ...
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What would you see standing on black hole just above the event horizon ? Will you look through past and future of the universe? [duplicate]

I am trying to imagine view from my house window if it was built on the black hole;) .... Of course I am joking. If time near event horizon slows down with respect to rest of the universe. I could ...
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Do the LIGO observations constitute proof of a black hole merger, and what happened to the black holes?

Information says that the gravitational waves, recently detected by LIGO, correspond (according to Einstein theory equations) to the effect of two black holes merging (with the masses of both black ...
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Electromagnetic four-potential for Kerr-Newman solution

Any textbook or paper on the Kerr-Newman metric I found contains the solution for the electromagnetic tensor $ F^{\mu}_{\phantom{\mu} \nu}$. Can you provide a (reliable) reference for the solution ...
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STU-model for Black holes

In string theory in the realm of black holes, what does the acronym STU stands for when we talk about the STU model?
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Complex tetrad vs. Real metric

I asked this question almost a month ago on mathoverflow (http://mathoverflow.net/q/228138/) but received no response. I thought I could have better luck here: I have a question on the relationship ...
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Blackhole microstate counting

It's widely belived that p-branes and Dp-branes are the same objects in string theory. P-branes are charged solutions of type IIB supergravity that have the topology of a black hole, one would expect ...
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Binary black hole system, near-field geodesics and post-newtonian formalism

I am interested in simulating the following system: Particle trajectory near one black hole (around 6-10 Schwarzschild radius) in the presence of another black hole (more than 50 Schwarzschild radius ...
2
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Merging and Splitting of Black Holes [duplicate]

We know that two black holes can merge to form a single black hole with the release of vast amounts of energy. Can the reverse happen? That is can it happen that large energy is supplied to a black ...
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Lense-Thirring Precession of short GRBs

Stone et al. 2014 have proposed that the jet resulting from the merger of a black hole (BH) and neutron star (NS) may precess due to Lense Thirring torques A large rotating mass e.g. the Earth, can ...
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Black Holes and the Multiverse [closed]

Are there any peer-reviewed theories which explore the idea that black holes in our universe may spawn other universes - or that our universe came from a black-hole-like event in another universe? ...
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283 views

How were the solar masses and distance of the GW150914 merger event calculated from the signal?

The GW150914 signal was observed, giving us the frequency and amplitude of the event. Because LIGO has two detectors a rough source location could be derived. But how do these three factors allow for ...
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Gravitation, light and black holes

I have few basic misunderstandings about black holes. I know that heavy mass causes a curved space time around it, what i don't understand is if this mass is a star and it becomes a black hole, why ...
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Black hole's singularity: Does it has to be multi-dimensional?

We assume that there is an infinite density at the center of a black hole. But we also know that if it was really infinite, it would apply an infinite gravitational force to masses even if they were ...
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Can the event horizon save conservation laws for black holes?

How reasonable it it to conclude that, from a remote observer’s frame, matter falling towards a black hole never crosses the event horizon, because ∆ t → 0 as v → c (according to the Lorentz ...
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When does a singularity start to exist during a black hole formation?

Excuse me if the question is naïve. I am not a physicist. Let's imagine formation of a black hole from a homogeneous collapsing star. At certain moment when enough of matter is inside of a small ...
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Since the horizon of a Nonstationary isolated black hole is a closed surface, how can gravitational waves be generated?

I understand this is from numerical calculations in GR. I have not seen them. Can anyone elucidate on what generates the GW waves - I know that deformities (anything moments higher than dipole) will ...
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What are black holes made of, and if one “eats” enough, will it cease being a black hole?

I kind of already know the answer to the second part (which must be 'yes'), but I'd still like to know what black holes essentially are. Are they made out of a special material to have density as high ...
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Interior material and spacetime of a black hole described from outside

This question is mostly about general relativity. Answers regarding the quantum physics behind it are also appreciated, but first I would prefer solutions from a relativistic viewpoint. If you ...
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Speed of stuff falling into a black hole, beyond the event horizon [duplicate]

We know the escape velocity from the event horizon (EH) $= c$. Anything that crosses the EH is destined to reach the singularity. Whether the escape velocity (beyond the EH) can be calculated, or not, ...
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What happens to a particle in the exact center of a Kerr black hole?

Kerr black holes (and Kerr-Newman black holes), instead of the "point" singularity theorized in spherically symmetric black holes, instead have a "ring" singularity, spread along the equatorial plane ...
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Do magnetic fields exist beyond the gravity of a black hole?

http://www.sciencealert.com/the-magnetic-field-just-outside-our-black-hole-has-been-studied-for-the-first-time reported strong magnetic fields escape out of black holes. Does that rule out photons ...
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Does anything exist between singularity and the event horizon of a black hole

It is said that anything that falls into the event horizon, quickly reaches the singularity. That would imply - there should be nothing between event horizon, and the singularity. Is that true? If ...
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A ball in the gravity potential field of a black hole — seems a paradox

As illustrated in the following diagram (A, B, C, D are 4 specified space points, and C is close to a black hole), a small ball at distance of a black hole is stationary (suppose now it's mass is m0) ...
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Event horizon, and Hawking radiation [duplicate]

When the escape velocity from EH is c, then how can Hawking radiation escapes it? Does it travel faster than c? Or, it escape from outside the EH? If so, how a BH can evaporate beyond EH?
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LIGO detection: What do we know about the black hole system?

I would to know how much we can infer from the LIGO detection on the black hole system. I understand that at least the following can be verified: Spins Inclination Initial masses Final masses ...
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Kerr Metric from rotated Schwarzschild?

Say we have got a system in GR that is described by the Schwazschild metric. Then we perform a coordinate transform that gives the metric in a rotating system. Why is the transformed metric not the ...